U.S. patent application number 11/493518 was filed with the patent office on 2006-11-23 for component of glass-like carbon for cvd apparatus and process for production thereof.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho(Kobe Steel, Ltd.). Invention is credited to Maki Hamaguchi.
Application Number | 20060261039 11/493518 |
Document ID | / |
Family ID | 31719838 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060261039 |
Kind Code |
A1 |
Hamaguchi; Maki |
November 23, 2006 |
Component of glass-like carbon for CVD apparatus and process for
production thereof
Abstract
Disclosed herein is an inner tube of glass-like carbon for CVD
apparatus and a process for production thereof. The inner tube has
its surface roughened without increase in metal impurities which
cause particles. It has improved adhesion to CVD deposit film and
also has a high degree of roundness. The surface roughness (on both
the inner and outer surfaces) is 0.1-10 .mu.m measured according to
JIS B0601. The concentration of metal impurities (iron, copper,
chromium, sodium, potassium, calcium, magnesium, and aluminum) in
the surface is less than 50.times.10.sup.10 atoms/cm.sup.2.
Inventors: |
Hamaguchi; Maki; (Kobe-shi,
JP) |
Correspondence
Address: |
C. IRVIN MCCLELLAND;OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko
Sho(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
31719838 |
Appl. No.: |
11/493518 |
Filed: |
July 27, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10631790 |
Aug 1, 2003 |
|
|
|
11493518 |
Jul 27, 2006 |
|
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Current U.S.
Class: |
216/88 |
Current CPC
Class: |
C04B 2235/725 20130101;
C04B 35/524 20130101; Y10T 428/30 20150115; C04B 2235/94 20130101;
C04B 2235/48 20130101; C23C 16/45591 20130101; C23C 16/4404
20130101; C04B 2235/65 20130101; C04B 2235/963 20130101; C04B
2235/9638 20130101; C23C 16/4581 20130101 |
Class at
Publication: |
216/088 |
International
Class: |
C03C 15/00 20060101
C03C015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 6, 2002 |
JP |
2002-229012 |
May 15, 2003 |
JP |
2003-137820 |
Claims
1-4. (canceled)
5. A process for producing a component of glass-like carbon, the
process comprising comprising steps of: molding a raw material
resin to produce a molded object having inner and outer surfaces,
carbonizing the molded object, roughening simultaneously the inner
and outer surfaces of the molded object, and purifying the molded
object after the carbonizing step.
6. The process as defined in claim 5, wherein the roughening step
is performed after any step between the molding step and the
carbonizing step.
7. The process as defined in claim 5, wherein the purifying step is
performed at any step after the roughening step.
8. The process as defined in claim 5, wherein the roughening step
comprises a mechanical surface roughening treatment.
9. The process as defined in claim 5, wherein the roughening step
comprises a mechanical surface roughening treatment and a chemical
surface roughening treatment; and the mechanical surface roughening
treatment is performed before the chemical surface roughening
treatment, or the mechanical surface roughening treatment and the
chemical surface roughening treatment are performed
simultaneously.
10. The process as defined in claim 8, wherein the mechanical
surface roughening treatment comprises sandblasting or
grinding.
11. The process as defined in claim 9, wherein the chemical surface
roughening treatment comprises thermal oxidation or electrolytic
oxidation.
12. The process as defined in claim 5, wherein the component of
glass-like carbon comprises a tube.
13. The process as defined in claim 5, wherein the purifying step
comprises heat treatment at a temperature of 2000.degree. C. or
above in a halogen-containing gaseous atmosphere.
14. (canceled)
15. The process as defined in claim 5, wherein the component of
glass-like carbon has a surface roughness (R.sub.a) ranging from
0.1 to 10 .mu.m (measured according to JIS B0601); and the
component of glass-like carbon contains in its surface iron,
copper, chromium, sodium, potassium, calcium, magnesium, and
aluminum each in an amount less than 5.times.10.sup.10
atoms/cm.sup.2.
16. The process as defined in claim 12, wherein the tube has a
maximum outside diameter and a minimum inside diameter; and a
difference between the maximum outside diameter and the minimum
inside diameter is 1.4 mm or less.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a component of glass-like
carbon for CVD (Chemical Vapor Deposition) apparatus. Although this
specification deals mainly with the inner tube for CVD apparatus,
the present invention also covers various components to be arranged
inside of CVD apparatus.
[0003] 2. Description of the Prior Art
[0004] Production of semiconductor devices generally relies on
so-called CVD process which involves gas-phase chemical reactions
of reactant gases to deposit silicon, silicon nitride, etc. in the
form of thin film on silicon wafers. In this process, silicon
wafers are placed in an inner tube (shown in FIG. 1) for their
uniform heating and controlled flow of reactant gases.
[0005] An inner tube for CVD apparatus is required to have good
durability (heat resistance at 500.degree. C. or above and
corrosion resistance to reactant gases) under ordinary CVD
conditions. It is also required to give off as little dust and
impurity gas as possible. These requirements have conventionally
been met by an inner tube made of quartz.
[0006] CVD process is based on the principle that heated reactant
gases (as raw materials) decompose or react with one another to
form film (such as polysilicon film and silicon nitride film) on
silicon wafers. During CVD process, decomposition or reaction
products of reactant gases deposit on the surface of the inner
tube. The inner tube carrying such deposits (polysilicon, silicon
nitride, etc.) is used repeatedly without replacement to maintain
productivity. After continued use, the inner tube becomes covered
with thick deposits, which eventually peel off from the inner tube
to give fine particles. Such fine particles stick to silicon
wafers, thereby reducing yields. Incidentally, "particles" denote
particulate defects which are detected when wafers are examined by
an optical tester.
[0007] One way to eliminate particles originating from deposit that
peels off from the inner tube is to periodically demount and clean
the inner tube to remove CVD deposit. Cleaning employs chemical
solutions like hydrofluoric acid and nitric acid.
[0008] Such cleaning operation, however, reduces productivity and
increases production cost. Therefore, the inner tube should permit
CVD deposit to firmly adhere to it so that accumulated CVD deposit
will not give rise to particles. Adhesion of CVD deposit to the
inner tube is affected by the dimensional change of the inner tube
which occurs when the CVD chamber is cooled and heated to unload
and load wafers, respectively. The inner tube should ensure good
adhesion despite its dimensional change, due to such
cooling/heating cycles. Moreover, since cleaning procedure to
remove CVD deposit is unavoidable, it is essential for the inner
tube to have good corrosion resistance to cleaning solutions as
mentioned above. Unfortunately, conventional inner tubes made of
quartz are insufficient in adhesion to CVD deposit and corrosion
resistance to cleaning solutions.
[0009] Under these circumstances, the present inventors previously
developed a new inner tube of glass-like carbon for CVD apparatus
which prevents the occurrence of particles, resists corrosion by
cleaning solutions, and meets other requirements mentioned above,
and they filed an application for patent (Japanese Patent Laid-open
No. 3504/2001).
[0010] There is another known way to improve adhesion to CVD
deposit for components of glass-like carbon or components of
graphite coated with glass-like carbon. It achieves its object by
blast finishing which roughens the surface of the component.
(Japanese Patent Laid-open No. 342068/2001 and Japanese Patent
Publication No. 86662/1994)
[0011] It is also known that the inner tube and other components
for CVD apparatus may contain impurities to contaminate wafers
during CVD process and thus induced contamination causes resulting
devices to malfunction. Therefore, the concentration of metal
impurities in the surface of components should be no more than
10.times.10.sup.10 atoms/cm.sup.2, preferably no more than
5.times.10.sup.10 atoms/cm.sup.2. In fact, a value lower than
5.times.10.sup.10 atoms/cm.sup.2 has been achieved in the case of a
commercial dummy wafer made of glass-like carbon prepared under
adequately controlled conditions. This dummy wafer has a
mirror-finished surface with a surface roughness no higher than 0.1
.mu.m. As with this dummy wafer, the inner tube of glass-like
carbon for CVD apparatus is required to have a surface metal
concentration no higher than 5.times.10.sup.10 atoms/cm.sup.2.
[0012] Incidentally, the surface metal concentration is determined
in the same way as used for determination of metal on the surface.
of silicon wafers. The procedure starts with extracting metal from
the sample with a mixed solution of 2% hydrofluoric acid and 2%
hydrogen peroxide. Then the extract is analyzed by ICP-MS
(inductively-coupled plasma mass spectrometry). The amount of metal
thus determined is expressed in terms of the number of atoms per
unit area of the sample used for extraction (atoms/cm.sup.2).
[0013] By the way, a tube of glass-like carbon is obtained by
preparing a resin tube and then heating it in an inert atmosphere.
The heating temperature is usually higher than 800.degree. C.,
preferably 1000-1200.degree. C., more preferably 1300-2500.degree.
C. During heating for carbonization, the resin tube greatly shrinks
(about 20% by volume). This shrinkage makes it difficult to obtain
a completely round tube of glass-like carbon.
[0014] There was proposed a method of achieving roundness by
inserting a cylindrical graphite core in a resin tube before
carbonization or in a glass-like carbon tube before heat treatment
at high temperatures, as disclosed in Japanese Patent Laid-open No.
189470/1999 and Japanese Patent Publication No. 189471/199. The
core has an outside diameter equal to the inside diameter which the
glass-like carbon tube would have as the result of the resin tube
shrinking after carbonization.
[0015] However, it turned out the above-mentioned conventional
method that utilizes a cylindrical core does not always give a
round glass-like carbon tube.
OBJECT AND SUMMARY OF THE INVENTION
[0016] The technology disclosed in Japanese Patent Laid-open No.
332504/2001 (mentioned above) permits one to effectively prevent
the occurrence of particles and to reduce the frequency of cleaning
procedures. This technology employs blast finishing to roughen the
surface of glass-like carbon inner tube for CVD apparatus to
improve adhesion of CVD deposits, however it does not reduce
particles to the required level. This is due to the fact that
surface roughening increases the content of metal impurities. At
present, there is a demand for an inner tube for CVD apparatus
which gives off less impurities than that achieved by the
above-mentioned technology.
[0017] Impurities such as particles occur when CVD deposit peels
off from the surface of the inner tube as mentioned above. The
present inventors, investigation revealed that the film of CVD
deposit becomes so thick as to cause cracking prior to peeling.
Cracks in the surface of CVD deposit film are shown in FIG. 5
(photograph taken by a scanning electron microscope).
[0018] Incidentally, CVD deposit film forms not only on the surface
of an inner tube but also on the surface of various components
exposed to CVD environment in CVD apparatus. Therefore, it is also
necessary to suppress impurities (particles) resulting from CVD
deposit film which forms on the surface of components other than
inner tube.
[0019] It is known that carbonaceous materials such as glass-like
carbon can be purified by heating at a high temperature (usually
2000.degree. C. or above) in a halogen-containing atmosphere, so
that halogen penetrates into carbon and vaporizes metal impurities
from carbon. This process seems useful in decreasing metal
impurities which may have contaminated the surface of an inner tube
at the time of surface roughening.
[0020] On the other hand, the purifying process may cause
dimensional distortion of the carbonaceous material. However, the
inner tube for CVD apparatus should maintain sufficient roundness
even after the purifying process. A possible reason why the
dimensional accuracy of glass-like carbon tube is not so high is
its anomalous volume change behavior. Namely, it shrinks up to
about 1200.degree. C., and then expands slightly at higher
temperatures.
[0021] Incidentally, as with CVD deposit film mentioned above,
metal impurities exist not only on the surface of the inner tube
but also on the surface of various components in the CVD
environment of CVD apparatus. Thus, it is necessary to reduce the
amount of metal impurities existing on the surface of components
other than inner tube, thereby suppressing the formation of
particles due to metal impurities.
[0022] Accordingly, it is an object of the present invention to
provide a component of glass-like carbon for CVD apparatus, which
offers the following advantages, and a process for production
thereof. Improved adhesion to CVD deposit. Ability to grow CVD
deposit film sufficiently thick before cracking, thereby
suppressing the occurrence of impurities (particles) that results
from CVD deposit film peeling off. Adequately roughened surface
which is made without increasing metal impurities that give rise to
particles. Ability to prevent itself from giving off dust. It is
another object of the present invention to provide a process for
producing a glass-like carbon tube having a high degree of
roundness.
[0023] The gist of the present invention resides in a component of
glass-like carbon for CVD apparatus which is characterized by
having a value of surface roughness (R.sub.a) ranging from 0.1 to
10 .mu.m (measured according to JIS B0601) and containing in its
surface iron, copper, chromium, sodium, potassium, calcium,
magnesium, and aluminum each in an amount less than
5.times.10.sup.10 atoms/cm.sup.2. The surface roughness (R.sub.a)
specified in the present invention is a value measured according to
JIS S0601, unless otherwise mentioned.
[0024] According to the present invention, the component of
glass-like carbon for CVD apparatus has a surface finished such
that there exist at least five pits, 1-10 .mu.m in diameter, in the
visual field, 50.times.50 .mu.m, observed under a scanning electron
microscope with a magnification of .times.1000, or a surface
finished such that there exist elongated tiny depressions, 0.5-5
.mu.m wide, whose total length is at least 50 .mu.m, in the visual
field, 50.times.50 .mu.m, observed under a scanning electron
microscope with a magnification of .times.1000. Incidentally, the
term "total length" means a value of the sum of the lengths of
elongated tiny depressions (0.5-5 .mu.m wide) present in the visual
field.
[0025] According to the present invention, the component of
glass-like carbon for CVD apparatus is any of inner tube, wafer
boat, susceptor, and nozzle used for CVD apparatus.
[0026] According to the present invention, the process for
producing the component of glass-like carbon for CVD apparatus is
characterized by a surface roughening step and an ensuing purifying
step. The surface roughening step may be a mechanical one, such as
sand blasting and grinding. The purifying step may be
high-temperature heat treatment in a halogen-containing
atmosphere.
[0027] In the case where the component of glass-like carbon for CVD
apparatus is a glass-like carbon tube, it is desirable that the
mechanical surface roughening be performed on both the inner and
outer surfaces and then the purifying step be carried out.
[0028] According to the present invention, the process for
producing a component of glass-like carbon for CVD apparatus is
characterized by mechanical surface roughening and chemical surface
etching, which may be performed sequentially (in the order
mentioned) or simultaneously.
[0029] The mechanical surface roughening may be accomplished by
sand blasting or grinding, and the chemical surface etching may be
accomplished by thermal oxidation or electrolytic oxidation.
[0030] According to the present invention, a round tube of
glass-like carbon for CVD apparatus is obtained by steps of molding
a raw material resin into a tube, heating the resulting resin tube
at 800-1300.degree. C., thereby converting it into a glass-like
carbon tube, and heating the glass-like carbon tube at a
temperature higher than 1500.degree. C., together with a roundness
correcting jig attached to the outside of the glass-like carbon
tube.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic diagram illustrating a vertical
low-pressure CVD apparatus, which is one of the CVD apparatus for
producing semiconductor devices by CVD process.
[0032] FIG. 2 is a schematic diagram illustrating the centrifugal
molding which is employed in the present invention.
[0033] FIG. 3 is a photograph taken by a scanning electron
microscope which shows tiny pits existing in the surface of the
inner tube for CVD apparatus which is covered by the present
invention.
[0034] FIG. 4 is a photograph taken by a scanning electron
microscope which shows elongated tiny depressions existing in the
surface of the inner tube for CVD apparatus which is covered by the
present invention.
[0035] FIG. 5 is a photograph taken by a scanning electron
microscope which shows cracks in the surface of nitride film formed
on the surface of an inner tube for CVD apparatus.
[0036] FIG. 6 is a graph showing the relation between the change in
length and the temperature of a piece of glass-like carbon which
have been carbonized at 900.degree. C.
[0037] FIG. 7 is a sectional view showing how a core (with carbon
fiber felt) is placed in a tube of phenolic resin.
[0038] FIGS. 8(a) and 8(b) are respectively a plan view and a
sectional view taken along A-A, which illustrate a roundness
correcting jig used in the present invention.
[0039] FIG. 9 is a sectional view showing a glass-like carbon tube
(before high-temperature heat treatment) with a roundness
correcting jig attached to its outside.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] According to the present invention, the inner tube of
glass-like carbon for CVD apparatus is characterized by having its
inner and outer surface roughened, so that it has improved adhesion
to CVD deposit owing to the anchoring effect produced by surface
irregularities. According to the present invention, the inner tube
of glass-like carbon for CVD apparatus should have a value of
surface roughness (R.sub.a) ranging from 0.1 to 10 .mu.m (measured
according to JIS B0601).
[0041] With a value of surface roughness smaller than 0.1 .mu.m,
the inner tube of glass-like carbon does not have sufficient
adhesion to CVD deposit, with the result that CVD deposit suffers
microcracking before it becomes sufficiently thick. This
microcracking gives off a large number of particles, thereby making
it necessary to replace the inner tube frequently. With a value of
surface roughness larger than 10 .mu.m, the inner tube of
glass-like carbon tends to lose its surface layer.
[0042] According to the present invention, the inner tube of
glass-like carbon for CVD apparatus has its inner and outer
surfaces roughened (with R.sub.a ranging from 0.1 to 10 .mu.m) by
surface roughening treatment. The surface roughening treatment
should preferably be mechanical one. Particularly, sand blasting
with fine powder is preferable to grinding, because it can achieve
surface roughening without causing surface defects such as
microcracking.
[0043] According to the present invention, the inner tube of
glass-like carbon for CVD apparatus is made of glass-like carbon,
so that it has a coefficient of thermal expansion in the
neighborhood of 3.times.10.sup.-6. This value is close to that of
silicon nitride (3.4.times.10.sup.-6). This means that there is no
significant dimensional change (and hence stress) due to heating
between the inner tube and the silicon nitride deposit in the
process of forming silicon nitride film. Such small stress
contributes to improved adhesion between the inner tube and the
silicon nitride deposit.
[0044] According to the present invention, the inner tube of
glass-like carbon for CVD apparatus is characterizing by containing
in its surface iron, copper, chromium, sodium, potassium, calcium,
magnesium, and aluminum each in an amount less than
5.times.10.sup.10 atoms/cm.sup.2. These metals are impurities
existing in the surface of the inner tube. If the concentration of
any of these metals exceeds 5.times.10.sup.10 atoms/cm.sup.2, the
inner tube tends to give off a large number of particles resulting
from these metals.
[0045] The present inventors continued their research on the cause
of impurities (such as particles). As the result, it was found that
the inner tube of glass-like carbon for CVD apparatus is not
satisfactory even though it has its inner and outer surfaces
roughened to an average roughness (R.sub.a) ranging from 0.1 to 10
.mu.m (measured according to JIS B0601). To be more specific, it
was found that the surface roughened by sand blasting does not
sufficiently improve adhesion to CVD deposit film. It was also
found that sand blasting leaves on the surface ceramics fine powder
or metal fine powder (used as abrasive media) or carbon fine powder
(given off by sand blasting) or stress (generated during sand
blasting). They are the cause of particles falling on wafers.
[0046] The foregoing suggests that, in order to completely
eliminate impurities such as particles, it is necessary to contrive
a method of improving adhesion to CVD deposit film more effectively
than sand blasting and a method of preventing the occurrence of
dust from the inner tube itself.
[0047] The present inventors' further investigation revealed that
it is possible to greatly improve adhesion between the inner tube
and CVD deposit film if the above-mentioned anchoring effect is
enhanced by mechanical surface roughening (such as sand blasting
that produces minute surface irregularities) and ensuring chemical
surface etching which forms extremely small surface irregularities
with very little change in surface roughness (R.sub.a). In what
follows, "adhesion" occasionally implies adhesion between the inner
tube surface and CVD deposit film.
[0048] The inner tube according to the present invention should
preferably have extremely small surface irregularities on its
surface as mentioned above. The surface irregularities are
specified by (1) or (2) below so that adhesion between the inner
tube and CVD deposit film is improved. Improved adhesion permits
the CVD deposit film to become sufficiently thick before it suffers
cracking. This prevents the occurrence of impurities such as
particles. [0049] (1) The inner tube should have on its surface at
least five pits, 1-10 .mu.m in diameter, in the visual field,
50.times.50 .mu.m, observed under an SEM with a magnification of
.times.1000. [0050] (2) The inner tube should have on its
surface-elongated tiny depressions, 0.5-5 .mu.m wide, whose total
length is at least 50 .mu.m, in the visual field, 50.times.50
.mu.m, observed under an SEM with a magnification of
.times.1000.
[0051] "Pits" in (1) above denotes approximately round depressions
existing on the surface of the inner tube, as shown in the SEM
photograph in FIG. 3. They are indicated by arrows in FIG. 3(b).
Their depth does not matter. ("Pits" may occasionally be referred
to as "surface holes".)
[0052] "Elongated tiny depressions" in (2) above are what is formed
after "pits" have grown. They are shown in an SEM photograph in
FIG. 4. In FIG. 4(c), they are marked with a thick line for easy
recognition.
[0053] The number of pits (surface holes) and the total length of
elongated tiny depressions, which are defined in the present
invention, are an average value of three measurements obtained from
a visual field of 50.times.50 .mu.m, with a magnification mentioned
above.
[0054] The surface structure specified in (1) above is such that
there exist at least five pits, 1-10 .mu.m in diameter, in the
visual field defined above. Such pits greatly improve adhesion
between the inner tube and CVD deposit film.
[0055] Pits less than five in the visual field are not enough to
produce a satisfactory anchoring effect (for adhesion between the
inner tube and CVD deposit film). The number of pits should
preferably be more than 10.
[0056] The number of pits in the visual field is not specifically
limited; however, it should preferably be less than 100 for the
reasons given below. Forming more than 100 pits requires surface
chemical etching for a long time. Also, as the number of pits
increases, the inner tube becomes thin without significant
improvement in adhesion. Thus, increasing the number of pits unduly
is not desirable from the industrial point of view.
[0057] Pits on the surface should have a diameter of 1-10 .mu.m as
specified in (1) above for the reasons given below. With a diameter
smaller than 1 .mu.m, pits do not produce the anchoring effect for
improvement in adhesion to CVD deposit film. On the other hand,
pits with a diameter larger than 10 .mu.m produce an adverse effect
on adhesion. Presumably, this is because each pit has a relatively
smooth surface and hence a large pit does not produce the anchoring
effect as desired.
[0058] Elongated tiny depressions (0.5-5 .mu.m wide) on the surface
should have a total length of at least 50 .mu.m as specified in (2)
above. If the total length is larger than 50 .mu.m, elongated tiny
depressions greatly improve adhesion between the inner tube and CVD
deposit film.
[0059] If the total length is smaller than 50 .mu.m, elongated tiny
depressions do not fully improve adhesion (due to anchoring effect)
between the inner tube and CVD deposit film. The total length
should preferably be larger than 100 .mu.m.
[0060] The total length of elongated tiny depressions does not have
an upper limit which is specifically established for improvement in
adhesion. However, elongated tiny depressions having a total length
exceeding 500 .mu.m do not produce any significant additional
effect. Therefore, the total length of elongated tiny depressions
should be less than 500 .mu.m from the industrial point of
view.
[0061] Elongated tiny depressions should have a width ranging from
0.5 to 5 .mu.m as specified in (2) above for the reasons given
below. Elongated tiny depressions with a width smaller than 0.5
.mu.m are too narrow to produce the anchoring effect for improved
adhesion to CVD deposit film. On the other hand, elongated tiny
depressions with a width larger than 5 .mu.m causes the inner tube
surface to give off fine powder.
[0062] As mentioned above, the inner tuber of the present invention
should have either or both of the surface structure specified in
(1) and (2) above.
[0063] The inner tube according to the present invention is
produced by the process explained in the following. A tubular form
of glass-like carbon, which is used as the inner tube in the
present invention, can be produced by an ordinary process, which
comprises of a step of molding a raw material resin and a step of
carbonizing the resulting molded object. The carbonizing step may
be preceded by a preliminary heating step to avoid distortion. In
the case where a thermosetting resin (mentioned later) is used as a
raw material resin, the curing treatment may function as the
preliminary heating step. In this way it is possible to cure the
resin constituting the molded object and then carbonize the resin
without serious thermal deformation.
[0064] To produce a tube of glass-like carbon, the molding step is
carried out so that the raw material resin is made into a hollow
cylindrical form. The molding method in this case is not
specifically restricted; it includes centrifugal molding, injection
molding, and extrusion molding. Of these molding methods,
centrifugal molding is particularly desirable for the reasons given
below. Centrifugal molding causes a molten resin to flow and cure
inside a mold; therefore, it readily yields tubular products with
high dimensional accuracy and it permits complete degassing
(because both ends of the mold are open at the time of molding).
Incidentally, the raw material resin includes any known
thermosetting resins such as phenolic resin and furan resin.
[0065] Details about centrifugal molding for raw material resin are
disclosed in Japanese Patent Laid-open No. 332504/2001.
[0066] The molding step to give a molded object (or resin tube) as
mentioned above is followed by the carbonizing step which converts
the resin tube into a glass-like carbon tube. Carbonization is
usually accomplished by heating at 800-2500.degree. C. in an inert
gas atmosphere (or non-oxidizing atmosphere). The carbonizing step
should be carried out in such a way that the glass-like carbon tube
maintains its round cross section. This object is achieved by using
a cylindrical core which is disclosed in Japanese Patent Laid-open
No. 179463/2002 and Japanese Patent Application No. 347393/2001.
The method of improving roundness will be mentioned later.
[0067] In the case where a thermosetting resin is used as a raw
material resin, it is desirable to provide a curing step as
mentioned above. The curing conditions vary from resin to resin; a
phenolic resin is usually cured in the air at 180-350.degree. C.
for 10-100 hours.
[0068] The inner tube according to the present invention may be
produced by a method which includes, in addition to the molding and
carbonizing steps, a mechanical surface roughening step and a
purifying step.
[0069] The mechanical surface roughening step is intended to form
minute irregularities on the surface of the glass-like carbon tube
by any known mechanical means. This step may be provided at any
stage between the molding step and the carbonizing step. Mechanical
surface roughening may be performed on the molded object of raw
material resin before or after curing or on the glass-like carbon
tube after carbonization. Incidentally, carbonization may be
accomplished in two stages at about 900.degree. C. and about
1200.degree. C. In this case, mechanical surface roughening may be
accomplished after the first stage or the second stage.
[0070] The method for mechanical surface roughening includes sand
blasting and grinding. Sand blasting is preferable.
[0071] Abrasive powder for sand blasting is not specifically
restricted, and any known one can be used. It includes ceramics
powder (such as alumina powder and silicon carbide powder), metal
powder, and glass beads. The grain size of abrasive powder and the
conditions of blasting (pressure and nozzle-work distance) may be
properly selected according to the surface roughness required. The
abrasive powder is usually fine powder with a grain size of about
#220-800.
[0072] Grinding may be accomplished by using sand paper
(#150-1000). The grain size and other conditions (grinding pressure
etc.) may be properly selected according to the surface roughness
desired.
[0073] Mechanical surface roughening should preferably be performed
on the inner and outer sides of the tube simultaneously so as to
minimize the dimensional change caused by the purifying step which
is carried out later. If mechanical surface roughening is performed
on the inner and outer sides separately, a comparatively large
stress occurs in the subsequent purifying step, and this makes it
difficult to obtain an inner tube with sufficient roundness for CVD
apparatus. (The reason for this is not known.)
[0074] According to another preferred embodiment, mechanical
surface roughening is accompanied by or followed by chemical
surface etching. Chemical surface etching is intended to partly
remove the surface of the glass-like carbon tube, thereby forming
minute surface irregularities. This treatment removes fine powder
and damages (such as cracking) resulting from mechanical surface
roughening. Therefore, it prevents the inner tube from releasing
dust.
[0075] Chemical surface etching is usually carried out after
mechanical surface roughening. If this order is reversed, minute
irregularities formed by chemical surface etching are destroyed by
mechanical surface roughening. To be more specific, chemical
surface etching is performed on the glass-like carbon tube on which
minute irregularities have been formed by mechanical surface
roughening in any stage. However, it is possible to perform
mechanical surface roughening (such as sand blasting) on the
glass-like carbon tube (which has not yet undergone mechanical
surface roughening) in an environment for chemical surface
roughening (for example, in an environment for thermal oxidation).
The result in this case is that mechanical surface roughening and
chemical surface roughening are carried out simultaneously.
[0076] The treatment for chemical surface roughening includes
thermal oxidation, electrolytic oxidation, and chemical
etching.
[0077] Thermal oxidation is carried out in such a way that the
surface of the glass-like carbon tube is oxidized to give the
surface structure as defined in (1) or (2) above. It is usually
accomplished by heat treatment in an oxidizing atmosphere (air or
oxygen) at 600-800.degree. C. for 0.5-10 hours.
[0078] Electrolytic oxidation is accomplished by applying electric
current across the glass-like carbon tube (as an anode) and a
cathode of platinum, stainless steel, or nickel, which are immersed
in an electrolyte. The electrolyte is an aqueous solution of sodium
hydroxide, calcium hydroxide, or ammonia. The concentration of the
electrolyte (NaOH solution) is 0.1-2 M.
[0079] Electrolytic oxidation may be carried out under any
unrestricted conditions so long as the resulting glass-like carbon
tube has the surface structure as defined in (1) or (2) above. The
degree of etching is controlled by adjusting the amount of current,
which is usually 5-500 C/cm.sup.2.
[0080] Chemical etching is accomplished by immersing the glass-like
carbon tube in a chemical solution capable of dissolving its
surface. The chemical solution is exemplified by potassium or
sodium dichromate aqueous solution and chromic acid mixture.
[0081] Electrolytic oxidation is preferable because it permits the
degree of etching to be controlled easily.
[0082] Incidentally, according to the present invention, the
glass-like carbon tube which has undergone mechanical surface
roughening alone or in combination with chemical surface etching
should have a value of surface roughness ranging from 0.1 to 10
.mu.m, preferably from 0.3 to 3 .mu.m.
[0083] The glass-like carbon tube which has undergone surface
roughening subsequently undergoes purifying treatment by heating at
a high temperature in a halogen-containing gas environment. This
purifying treatment reduces the content of the metal impurities
which may have entered the surface of the glass-like carbon tube at
the time of mechanical surface roughening. After this purifying
treatment, followed by surface cleaning treatment, the
concentration of each of iron, copper, chromium, sodium, potassium,
calcium, magnesium, and aluminum in the surface is reduced to less
than 5.times.10.sup.10 atoms/cm.sup.2.
[0084] The present inventors' investigation revealed that if the
surface roughness of the inner tube exceeds 10 .mu.m, the purifying
treatment does not reduce the metal concentration below
5.times.10.sup.10 atoms/cm.sup.2, even after the surface cleaning
treatment. A probable reason for this is that excessive surface
roughening increases the effective surface area and hence the metal
concentration per unit area of the geometrical surface of the inner
tube increases. Therefore, the inner and outer surfaces of the
inner tube for CVD apparatus should be roughened to a certain limit
which is 10 .mu.m, from the standpoint of reducing the metal
concentration in the surface. Incidentally, after the purifying
treatment, the inner tube should be washed with hydrofluoric acid,
hydrochloric acid, or hydrogen peroxide in the usual way.
[0085] According to the present invention, the following method is
employed to improve the roundness of the glass-like carbon tube.
The process according to the present invention includes the steps
of molding a raw material resin into a tube and carbonizing the
resin tube by heating in an inert atmosphere at 800-1300.degree. C.
In the carbonizing step, the resin tube shrinks and vitrifies to
give a glass-like carbon tube which subsequently undergoes
high-temperature heat treatment. The carbonizing step should be
carried out in such a way that a cylindrical core is placed inside
the resin tube so that the tube remains round during heat
treatment. Then, a roundness correcting jig is place around the
outside of the glass-like carbon tube (which does not yet undergo
high-temperature heat treatment). The glass-like carbon tube and
the roundness correcting jig are heated together at a high
temperature (say, 1500.degree. C. or above) in an inert atmosphere.
During heat treatment at such a high temperature, the roundness
correcting jig undergoes thermal expansion and the glass-like
carbon tube also undergoes thermal expansion as well as expansion
due to structural change of carbon. As the result, the glass-like
carbon tube, which has undergone high-temperature heat treatment,
keeps a high degree of roundness, with its outer surface conforming
to the inner surface of the roundness correcting jig.
[0086] FIG. 6 is a graph showing the relation between the
temperature and the change in length in the case where the
glass-like carbon sample (which does not yet undergo
high-temperature heat treatment) is heated. The rate of heating is
200.degree. C./h. The glass-like carbon sample is a rectangular
shape measuring 2 mm thick, 2 mm wide, 20 mm long, which has been
carbonized by heating at 900.degree. C.
[0087] As FIG. 6 shows, the molded sample of glass-like carbon
expands until it is heated to about 950.degree. C., shrinks while
it is heated from 950.degree. C. to 1200.degree. C., and expands
again while it is heated above 1200.degree. C. The slope of the
second expansion (at 1200.degree. C. and above) is steeper than
that of the first expansion (at 0-900.degree. C.). At 0-900.degree.
C., the molded sample of glass-like carbon has a coefficient of
linear expansion of about 3.times.10.sup.-6 (K.sup.-1), while at
1200-1500.degree. C., it has a coefficient of thermal expansion of
about 10.times.10.sup.-6 (K.sup.-1) The difference between the two
coefficients is attributable to the change in structure that occurs
at 1200-1500.degree. C. In other words, the molded sample of
glass-like carbon (which has been carbonized at 900.degree. C. in
this example) expands when it is heated from 1200.degree. C. to
1500.degree. C. The mechanism of expansion is not fully elucidated;
probably it is caused by the change in structure of carbon.
[0088] The foregoing suggests that carbonization to give the
glass-like carbon tube (which subsequently undergoes
high-temperature heat treatment) should be carried out at lower
than 1200.degree. C., so that the glass-like carbon tube is heated
for a long time at 1200-1500.degree. C. in the high-temperature
heat treatment. The upper limit of the carbonizing temperature
should be 1300.degree. C., with possible variation of raw material
resin taken into consideration. The lower limit of the carbonizing
temperature should be 800.degree. C. at which the resin tube
vitrifies. This lower limit should preferably be as close to
1200.degree. C. as possible at which the material shrinks almost
completely. (A molded object of glass-like carbon which has been
heated at 900.degree. C. shrinks at 950-1200.degree. C.) The reason
for this is that if the glass-like carbon tube greatly shrinks
while it is heated through a temperature range from 950 to
1200.degree. C., the glass-like carbon tube excessively separates
from the roundness correcting jig surrounding it and this lessens
the subsequent roundness correcting effect due to expansion.
Incidentally, the reason why the high-temperature heat treatment
should be carried out at 1500.degree. C. or above is that it does
not fully produce the roundness correcting effect due to expansion
(resulting from the structural change mentioned above) if its
temperature is lower than specified. The upper limit of the
temperature for high-temperature heat treatment is usually
2500.degree. C. although it varies depending on the quality and
usage temperature of the glass-like carbon tube desired.
[0089] The roundness correcting jig used in the present invention
should be formed from a material having good heat resistance and a
coefficient of thermal expansion close to that of the glass-like
carbon tube which undergoes high-temperature heat treatment. The
material includes graphite, glass-like carbon, and carbon fiber,
with the first being preferable.
[0090] The foregoing description is directed mostly to an inner
tube of glass-like carbon for CVD apparatus. The scope of the
present invention covers not only inner tubes but also other
components for CVD apparatus, such as boats to hold wafers,
susceptors, nozzles, etc. They are prepared from a raw material
resin by molding, carbonization, surface roughening, and surface
chemical etching as in the case of inner tube.
[0091] As explained above for inner tubes, these components of
glass-like carbon for CVD apparatus have extremely tiny surface
irregularities due to mechanical surface roughening and extremely
small surface irregularities due to chemical surface etching. This
surface structure produces the anchoring effect, contributing to
improved adhesion to CVD deposit film. In addition, the chemical
surface etching prevents the components from releasing dust from
themselves. These properties are very useful in production of
semiconductor devices.
EXAMPLES
[0092] The invention will be described in more detail with
reference to the following examples, which are not intended to
restrict the scope thereof and changes and modifications may be
made without departing from the scope thereof.
[0093] FIG. 2 is a schematic sectional view showing an example of
the mold for centrifugal molding which is used in one example of
the present invention. The mold 10 for centrifugal molding consists
of a mold proper 11, a bottom plate 12, and a flange stopper 13.
The mold proper 11 is a cylindrical body of stainless steel,
measuring 325 mm in inside diameter and 1600 mm long. It has a
detachable bottom plate 12 on one end thereof, which is driven by a
motor through a belt. It also has a detachable annular flange
stopper 13 on the other end thereof, which prevents the
thermosetting resin from leaking therefrom. This flange stopper 13
has an opening which permits reaction gases to escape. The mold
proper 11 is of separable type (consisting of two sections) for
easy demolding. Centrifugal molding is accomplished by charging the
mold proper 11 with a thermosetting resin and then rotating the
mold assembly 10 while heating it at a thermosetting temperature.
Heating is accomplished by electric heaters surrounding the mold
proper 11.
Example 1
Preparation of Glass-Like Carbon Tube
[0094] Raw material resin: A preferred one for glass-like carbon is
a thermosetting resin such as phenolic resin and furan resin. This
example employed a commercial phenolic resin "PL4804" from Gun-ei
Kagaku Kogyo Co., Ltd., which had previously been vacuum-dried
(below 5 wt % water content) at 65.degree. C. for 6 hours under a
reduced pressure of 10 Torr. Preparation of phenolic resin tube: A
tube of phenolic resin was formed as follows by using a centrifugal
molding machine equipped with the mold 10. The mold 10 was charged
with the phenolic resin in a prescribed amount. The mold 10 was
rotated at 600 rpm for 5 hours, with its surface temperature kept
at 100.degree. C. The mold was cooled to room temperature for
demolding. Thus there was obtained a phenolic resin tube measuring
3 mm thick, 320 mm in outside diameter, and 1500 mm long. This
procedure was repeated to prepare several samples. [0095] Curing:
The thus obtained phenolic resin tube was cured by heating at
300.degree. C. for 200 hours. This curing is intended to prevent
the phenolic resin tube from deforming during carbonization;
therefore, it may be omitted if there is no possibility of
deformation. [0096] Carbonization: The phenolic resin tube was
carbonized by heating at 1600.degree. C. in an inert gas (nitrogen)
atmosphere. Thus there was obtained a glass-like carbon tube
measuring 2.5 mm thick, 268 mm in outside diameter, and 1265 mm
long. [0097] Sand blasting: The glass-like carbon tube thus
obtained was exposed to a jet stream of a slurry of silicon carbide
in water, which was directed to both inside and outside thereof, so
that both sides are roughened simultaneously. In this process, each
sample underwent roughening with silicon carbide varying in grain
size, with other conditions (pressure and nozzle-surface distance)
unchanged. [0098] Electrolytic oxidation: The glass-like carbon
tube with its surface roughened underwent electrolytic oxidation by
immersion in an aqueous solution of sodium hydroxide (0.1 mol/L),
with current applied to between the sample and a platinum sheet
counter electrode (cathode). This step was followed by washing and
drying in the usual way. [0099] Purifying process: The glass-like
carbon tube which had undergone electrolytic oxidation was heated
(for purifying) at 2200.degree. C. in a chlorine-containing
atmosphere. The purified sample was cleaned with a mixture of 2%
hydrofluoric acid and 2% hydrogen peroxide and then rinsed
repeatedly with ultrapure water and dried in a clean room.
Experiment Nos. 1 to 4
[0100] Four samples (Nos. 1 to 4) of inner tube for CVD apparatus
were prepared by performing the above-mentioned steps in the
following order. Preparation of phenolic resin
tube.fwdarw.Curing.fwdarw.Carbonization.fwdarw.Sand
blasting.fwdarw.Electrolytic oxidation.fwdarw.Purifying.
Experiments Nos. 5 and 6
[0101] Two samples (Nos. 5 and 6) of inner tube for CVD apparatus
were prepared by performing the above-mentioned steps in the
following order. Preparation of phenolic resin
tube.fwdarw.Curing.fwdarw.Carbonization.fwdarw.Purifying.fwdarw.Sand
blasting.fwdarw.Electrolytic oxidation. (Sand blasting for No. 5
and No. 6 was performed under the same condition as for No. 1 and
No. 2, respectively.) The procedure for Nos. 5 and 6 is the same as
that for Nos. 1 and 2 except that the purifying step precedes the
sand blasting and electrolytic oxidation steps.
Experiments Nos. 7 and 8
[0102] The sample No. 7 was prepared by the same procedure as for
samples Nos. 1 to 4, except that the sand blasting step for surface
roughening employed finer silicon carbide particles. The sample No.
8 was prepared by the same procedure as for samples Nos. 1 to 4,
except that the sand blasting step for surface roughening employed
coarser silicon carbide particles.
Experiment No. 9
[0103] The sample No. 9 was prepared by the same procedure as for
samples Nos. 1 to 4, except that the sand blasting step was
performed in two stages, the first one for inside and the second
one for outside.
[0104] The thus obtained samples Nos. 1 to 9 were tested for the
following items. The results are shown in Table 1. [0105] (1)
Surface roughness:
[0106] After surface roughening, the inner and outer surfaces of
the inner tube for CVD apparatus were tested for surface roughness
(R.sub.a) specified by JIS B0601 by using a contact probe-type
roughness tester (made by Rank Tailor Co., Ltd.) The surface
roughness (R.sub.a) is expressed in terms of an average value of
measurements made in two regions (in each of the inner and outer
surfaces) in which there are no scratches due to handling. [0107]
(2) Number of particles:
[0108] The number of particles released from the inner tube for CVD
apparatus was counted in the following manner. The inner tube was
placed in a vertical CVD apparatus which had been charged with
silicon wafers. The CVD apparatus was fed with a mixed gas of
dichlorosilane (SiCl.sub.2H.sub.2) and ammonia (NH.sub.3) at
770.degree. C. under reduced pressure. The step of forming silicon
nitride film (150 nm thick) on each silicon wafer was repeated 100
times. After the first run and the hundredth run, the number of
particles larger than 0.2 .mu.m was counted. This procedure was
accomplished by using a Surfscan (Model WH-1700, made by Topcon).
[0109] (3) Concentration of metal:
[0110] The concentration of metal in the sample was determined by
ICP mass spectrometry in the following manner. A small amount of a
50:50 mixture of 2% hydrofluoric acid and 2% hydrogen peroxide is
dropped onto the surface of the silicon wafer to be analyzed. The
solution on the surface is allowed to stand for about 10 minutes
and then recovered. The concentration of metal was analyzed by
ICP-MS, and the figures are expressed in terms of the number of
metal atoms per unit area of the silicon wafer. The chemicals used
for analysis was ultrapure reagents made by Kanto Kagaku Co., Ltd.
The apparatus used for ICP mass spectrometry was SPQ9000SE made by
Seiko Instruments Co., Ltd. The apparatus was run under the
condition (a) for iron, chromium, sodium, potassium, calcium,
magnesium, and aluminum, and the condition (b) for copper. [0111]
(a) high-frequency output: 0.8 kW, sampling depth: 8 mm, carrier
gas: 0.9 L/min, with PFA microflow nebulizer, chamber gas: 0.33
L/min. [0112] (b) high-frequency output: 1.3 kW, sampling depth: 8
mm, carrier gas: 0.9 L/min, with PFA microflow nebulizer, chamber
gas: non. [0113] (4) Roundness:
[0114] The maximum and minimum values of the outside diameter were
measured at the upper end of the inner tube for CVD apparatus.
Roundness was expressed in terms of difference between the maximum
outside diameter and the minimum outside diameter. TABLE-US-00001
TABLE 1 Surface Number of particles Experiment roughness Roundness
Concentration of metal in the surface (.times.10.sup.10
atoms/cm.sup.2) After After No. Ra (.mu.m) (mm) Fe Cu Cr Na K Ca Mg
Al 1.sup.st run 100.sup.th run 1 0.13 0.9 <0.2 <0.2 <0.2
<0.2 <0.2 0.9 0.3 1.1 38 73 2 1.3 0.8 <0.2 <0.2 <0.2
0.5 0.4 0.9 0.5 2.1 35 58 3 2.6 1.2 <0.2 <0.2 <0.2 1.6 1.9
1.8 1.1 3.1 39 48 4 7.9 0.7 <0.2 <0.2 <0.2 0.4 0.5 2.3 1.6
1.7 48 82 5 0.32 0.8 7.8 1.4 1.2 10.6 11.2 13.5 16.3 13.9 336 -- 6
1.3 0.8 5.9 2.4 1.6 13.7 10.5 12.1 11.6 16.2 415 -- 7 0.08 1.1
<0.2 <0.2 <0.2 <0.2 <0.2 0.3 1.2 3.8 46 -- 8 10.9
1.4 6.3 2.1 <0.2 15.8 9.9 13.4 16.1 13.8 442 -- 9 2.4 3.2 0.8
0.3 <0.2 0.3 0.6 1.9 2.2 0.9 -- --
[0115] As Table 1 shows, in the case of samples Nos. 1 to 4, which
have the surface roughness and the surface metal concentration
within the range specified in the present invention, the number of
particles was less than 60 after the first run of film forming
operation and even after the hundredth run of film forming
operation in which the cumulative film thickness reached 15 .mu.m.
This result is good for satisfactory device yields.
[0116] In contrast, samples Nos. 5 and 6, which underwent sand
blasting and electrolytic oxidation after the purifying step, were
found to have residual metal impurities which had entered the
surface of the glass-like carbon tube during its surface roughening
by sand blasting. Therefore, they failed to meet the requirement of
surface metal concentration specified in the present invention.
They gave off a large number of particles (presumably originating
from metal impurities) throughout the film forming process. They
were not suitable for production of semiconductor devices.
[0117] Sample No. 7, which underwent sand blasting with fine
silicon carbide grits, had a value of surface roughness which is
lower than that specified in the present invention. It gave off a
less number of particles in the initial stage of film forming
process; however, the number of particles increased as the film
forming process was repeated. It did not permit the film forming
process to be repeated 100 times owing to more particles than
specified. These excessive particles originated from microcracking
in the CVD deposit film which became thicker after the repeated
film forming process. Sample No. 8, which underwent sand blasting
with coarse silicon carbide grits, had a value of surface roughness
which is higher than that specified in the present invention.
Because of its large surface roughness (10.9 .mu.m) exceeding the
upper limit (10 .mu.m), it had a surface metal concentration in
excess of 5.times.10.sup.10 atoms/cm.sup.2 despite the purifying
process. It was not used for production of semiconductor devices
because it gave off a large number of particles in the first run of
film forming process.
[0118] Sample No 9, which was poor in roundness (3.2 mm in terms of
difference between the maximum and minimum outside diameters at the
top of the tube) due to a comparatively large strain experienced in
the purifying process, was not suitable for the use as an inner
tube of CVD apparatus because it took a very long time to be
machined into a proper dimension.
Example 2
Preparation of Glass-Like Carbon Tube
[0119] Samples of glass-like carbon tubes were prepared as follows
from a commercial phenolic resin "PL4804" from Gunei Kagaku Kogyo
Co., Ltd., which had previously been vacuum-dried (below 5 wt %
water content) at 100.degree. C. for 1 hour under a reduced
pressure of 10 Torr. The phenolic resin was made into a tube by
using a centrifugal molding machine equipped with the mold 10.
Charged with 7 kg of the phenolic resin, the mold 10 was turned at
600 rpm for 10 hours, with its surface temperature kept at
120.degree. C. so that the phenolic resin was melted. The mold was
cooled to room temperature for demolding. Thus there was obtained a
phenolic resin tube measuring 3 mm thick, 323 mm in outside
diameter, and 1590 mm long.
[0120] The phenolic resin tube was cured by heating in the air at
250.degree. C. for 10 hours. The cured phenolic resin tube was
carbonized by heating at 1600.degree. C. in an inert gas (nitrogen)
atmosphere. Thus there was obtained a glass-like carbon tube
measuring 2.5 mm thick, 268 mm in outside diameter, and 1265 mm
long.
Experiment No. 1
[0121] The sample of glass-like carbon tube mentioned above was
sand-blasted with #400 alumina powder for mechanical roughening of
the inner and outer surfaces thereof. After sand blasting, the
sample had a value of surface roughness of 0.6 .mu.m.
[0122] Subsequently, the sample underwent electrolytic oxidation
(for chemical surface etching) under the conditions shown in Table
2 in a 0.1M aqueous solution of sodium hydroxide, with current
applied to between the sample and a platinum sheet counter
electrode (cathode). This step was followed by washing and drying
in the usual way. The thus treated sample was found to have a value
of surface roughness of 0.6 .mu.m. In other words, the chemical
surface etching did not change surface roughness.
[0123] The sample which had undergone electrolytic oxidation was
purified by heating at 2200.degree. C. in a chlorine-containing
gas. The purified sample was washed with a mixture of 2%
hydrofluoric acid and 2% hydrogen peroxide and then rinsed with
ultrapure water repeatedly. It was finally dried in a clean room.
Thus there was obtained the desired inner tube for CVD apparatus.
It was examined for the surface metal concentration in the same way
as in Example 1. The result was no more than 5.times.10.sup.10
atoms/cm.sup.2 in all the samples.
Experiments Nos. 2 to 7
[0124] The samples underwent sand blasting in the same way as in
Experiment No. 1. After sand blasting, the samples were found to
have a value of surface roughness of 0.6 .mu.m. The samples
underwent electrolytic oxidation in the same way as in Experiment
No. 1, except that the conditions were changed as shown in Table 2.
Electrolytic oxidation was followed by washing, drying, and
purifying. Thus there were obtained inner tubes for CVD apparatus.
They were found to have a value of surface roughness of 0.6 .mu.m.
This indicates that the chemical surface etching (electrolytic
oxidation) did not change surface roughness. They were also
examined for the surface metal concentration in the same way as in
Example 1. The result was no more than 5.times.10.sup.10
atoms/cm.sup.2 in all the samples.
[0125] The thus obtained inner tubes for CVD apparatus (numbered 1
to 7) were tested for the following items. The results are shown in
Table 2. [0126] (1) Surface roughness:
[0127] The samples were tested for surface roughness in the same
way as in Example 1. [0128] (2) Pits and elongated tiny
depressions:
[0129] The samples were examined for their surface by observation
under an SEM (.times.1000). Three arbitrary visual fields
(50.times.50 .mu.m) were photographed. The number of pits (1-10
.mu.m in diameter) in each visual field was counted and the total
length of elongated tiny depressions (0.5-5 .mu.m wide) in each
visual field was measured. Average values per visual field were
calculated. [0130] (3) Thickness at which surface cracking occurs
in CVD deposit film (nitride film):
[0131] Small pieces of the inner tube were set in a vertical type
low pressure CVD apparatus. The CVD apparatus was fed with a
mixture gas of NH.sub.3 and SiCl.sub.2H.sub.2 at 780.degree. C. so
that nitride film was formed on the surface of the inner tube. The
surface of the nitride film formed on the pieces was observed under
an SEM (.times.5000) in arbitrary ten visual fields at certain time
intervals. The film thickness at which cracking began to occur was
recorded. [0132] (4) Number of particles:
[0133] The number of particles released from the inner tube for CVD
apparatus was counted in the following manner. The inner tube was
placed in a vertical CVD apparatus which had been charged with
silicon wafers. The CVD apparatus was heated to 800.degree. C.
under reduced pressure without being fed with reactant gases. The
wafers were removed from the CVD apparatus, and the number of
particles on them was counted.
[0134] Incidentally, FIG. 3 is an SEM photograph showing the
surface of the inner tuber for CVD apparatus which was produced
under the same condition as in Sample No. 4. FIG. 3(a) consists of
two parts. The part below the broken line represents the surface
which was masked at the time of chemical surface etching so that it
substantially underwent only mechanical surface roughening (sand
blasting). The part above the broken line represents the surface
which underwent mechanical surface roughening and subsequent
chemical surface etching. FIG. 3(b) is a partial enlargement
representing a visual field (50.times.50 .mu.m) in FIG. 3(a).
Arrows in FIG. 3(b) denote surface pits.
[0135] FIG. 4 is an SEM photograph showing the surface of the inner
tuber for CVD apparatus which was produced under the same condition
as in Sample No. 3. FIG. 4(b) is a partial enlargement representing
a visual field (50.times.50 .mu.m) in FIG. 4(a). Elongated
depressions are apparently noticed. In FIG. 4(c) elongated
depressions are marked with thick lines for easy recognition.
TABLE-US-00002 TABLE 2 Thickness Amount of Total length at which
Number of Exper- current Number of elongated surface particles
iment applied of pits in depressions cracking (per No. (C/cm.sup.2)
surface (.mu.m) occurs (.mu.m) wafer) 1 10 4 55 10 115 2 40 7 60 12
97 3 80 10 260 18 53 4 100 17 -- * 22 62 5 200 32 -- * 38 49 6 0 0
0 0.4 233 7 5 3 20 1.3 196 Remarks: unmeasurable due to excessive
etching.
[0136] Samples Nos. 1 to 5, which meet the requirements for the
number of surface pits and the total length of elongated tiny
depressions, permitted nitride film to grow to a substantial
thickness before surface cracking occurs. This means that they can
be used continuously without dust release until a thick nitride
film is formed thereon. In addition, they gave off very few
particles during operation.
[0137] By contrast, samples Nos. 6 and 7, which do not meet the
requirements for the number of surface pits and the total length of
elongated tiny depressions, have the following shortcomings.
[0138] Sample No. 6, which did not undergo chemical surface etching
(electrolytic oxidation), caused the nitride film to crack soon
after the start of operation. Cracking of nitride film necessitates
replacement and cleaning of the inner tuber in the early stage and
also gives off many particles during operation.
[0139] Sample No. 7, which underwent electrolytic oxidation with a
reduced amount of current, has a less number of pits and a less
total length of elongated depressions. Therefore, it caused the
nitride film to crack soon after the start of operation. Cracking
of nitride film necessitates replacement and cleaning of the inner
tuber in the early stage and also gives off many particles during
operation.
Experiment No. 8
[0140] The glass-like carbon tube mentioned above underwent
grinding with sand paper (#240) for mechanical surface roughening
on its inner and outer surfaces. This treatment gave a surface
roughness of 2.1 .mu.m.
[0141] Subsequently, the glass-like carbon tube underwent thermal
oxidation in the air for 1 hour at varied temperatures shown in
Table 3. This process did not change the surface roughness (2.1
.mu.m).
[0142] The glass-like carbon tube, which had been thermally
oxidized, underwent purifying treatment, followed by cleaning and
drying, in the same way as in experiments Nos. 1 to 7. The
resulting inner tube for CVD was tested for surface metal
concentration in the same way as in Example 1. The result was less
than 5.times.10.sup.10 atoms/cm.sup.2 in all the samples.
Experiments Nos. 9 to 12
[0143] Samples of glass-like carbon tubes with grinding treatment
were prepared in the same way as in Experiment No. 8. They were
found to have a value of surface roughness of 2.1 .mu.m. They
underwent thermal oxidation and purifying process in the same way
as in Experiment No. 8, except that the temperature was changed as
shown in Table 3. The resulting inner tubes for CVD apparatus were
found to have a value of surface roughness of 2.1 .mu.m. This
suggests that chemical surface etching (thermal oxidation) did not
change surface roughness. They were also tested for surface metal
concentration in the same way as in Example 1. The result was less
than 5.times.10.sup.10 atoms/cm.sup.2 in all the samples.
[0144] The thus obtained inner tubes for CVD apparatus were tested
for the items in Experiments Nos. 1 to 7. The results are shown in
Table 3. TABLE-US-00003 TABLE 3 Thickness at which Temperature
Total length surface Number of Exper- of thermal Number of
elongated cracking particles iment oxidation of pits in depressions
occurs (per No. (.degree. C.) surface (.mu.m) (.mu.m) wafer) 8 650
6 58 8 92 9 675 21 114 16 87 10 700 29 -- * 26 89 11 -- 0 0 0.6 325
12 600 5 42 1.5 126 Remarks: unmeasurable due to excessive
etching.
[0145] Samples Nos. 8 to 10, which meet the requirements for the
number of surface pits and the total length of elongated tiny
depressions, permitted nitride film to grow to a substantial
thickness before surface cracking occurs. This means that they can
be used continuously without dust release until a thick nitride
film is formed thereon. In addition, they gave off very few
particles during operation.
[0146] By contrast, samples Nos. 11 and 12, which do not meet the
requirements for the number of surface pits and the total length of
elongated tiny depressions, have the following shortcomings.
[0147] Sample No. 11, which did not undergo chemical surface
etching (thermal oxidation), caused the nitride film to crack soon
after the start of operation. Cracking of nitride film necessitates
replacement and cleaning of the inner tuber in the early stage and
also gives off many particles during operation.
[0148] Sample No. 12, which underwent thermal oxidation at a lower
temperature, has a less number of pits and a less total length of
elongated tiny depressions. Therefore, it caused the nitride film
to crack soon after the start of operation. Cracking of nitride
film necessitates replacement and cleaning of the inner tuber in
the early stage and also gives off many particles during
operation.
Example 3
Experiment No. 1
[0149] Samples of glass-like carbon tubes were prepared as follows
from a commercial phenolic resin "PL4804" from Gunei Kagaku Kogyo
Co., Ltd., which had previously been dehydrated. The phenolic resin
was made into a tube by using a centrifugal molding machine
equipped with the mold 10. Charged with a prescribed amount of the
phenolic resin, the mold 10 was rotated at 600 rpm for 24 hours,
with its surface temperature kept at 100.degree. C. The mold was
cooled to room temperature for demolding. Thus there was obtained a
phenolic resin tube 21 measuring 2.5 mm thick, 330 mm in outside
diameter, and 1200 mm long.
[0150] Then, the phenolic resin tube 21, with a core 23 inserted
therein as shown in FIG. 7, was carbonized at 1200.degree. C. in
the following manner. (This core is covered with carbon fiber
felt.) The phenolic resin tube 21, together with the core 23, was
heated in an electric furnace filled with nitrogen gas, with the
temperature therein raised to 1200.degree. C. at a rate of
2.degree. C./h. The phenolic resin tube 21 was kept at 1200.degree.
C. for 1 hour for carbonization. The core 23 was removed. Thus
there was obtained a glass-like carbon tube 22 measuring 1.9 mm
thick, 249. mm in outside diameter (assuming a round tube), and 900
mm long. Incidentally, the core 23 is a graphite tube 23a,
measuring 15 mm thick, 240 mm in outside diameter, and about 1200
mm long. It has one layer of carbon fiber felt (3 mm thick) 23b
surrounding its outer surface, as shown in FIG. 7.
[0151] FIG. 8 shows one example of the roundness correcting jig
used in the present invention. FIG. 8(a) is a plan view, and FIG.
8(b) is a sectional view taken along the line A-A. The roundness
correcting jig is made of graphite. As shown in FIG. 8(a), it is a
square plate measuring 300.times.300 mm and 30 mm thick. It has a
round hole 24a, 250 mm in diameter, at its center. Into this hole
24a is inserted the glass-like carbon tube 22 mentioned above.
[0152] As FIG. 9 shows, the roundness correcting jig 24 was
arranged at the outside of one end of the glass-like carbon tube
22, and the resulting assembly was heated in an electric furnace
for high-temperature treatment in nitrogen gas. The temperature was
raised to 1200.degree. C. at a rate of 20.degree. C./h and then to
1500.degree. C. at a rate of 2.degree. C./h. This temperature was
kept for 1 hour. After this high-temperature treatment, the
roundness correcting jig 24 was removed.
[0153] The resulting glass-like carbon tube underwent sand blasting
with #400 alumina powder for mechanical surface roughening on both
the inner and outer surfaces thereof. Subsequently, it underwent
purifying treatment by heating at 2200.degree. C. in a
chlorine-containing gas atmosphere.
[0154] After purifying treatment, the glass-like carbon tube was
examined for roundness. It was found that there is a difference of
0.3 mm between the maximum outside diameter and the minimum outside
diameter measured at the end at which the roundness correcting jig
was arranged. The roundness correcting jig proved useful in
production of a large-scale glass-like carbon tube with a high
degree of roundness. The round glass-like carbon tube does not need
grinding for dimensional adjustment; it can be used as an inner
tube of CVD apparatus.
Experiment No. 2
[0155] A phenolic resin tube, measuring 2.5 mm thick, 330 mm in
outside diameter, and 1200 mm long, was prepared in the same way as
in Experiment No. 1. It underwent heat treatment, with a core 23
arranged therein, in an electric furnace filled with nitrogen gas.
The temperature was raised to 1500.degree. C. at a rate of
2.degree. C./h. This temperature was kept for 1 hour. The resulting
glass-like carbon tube underwent mechanical surface roughening and
purifying treatment in the same way as in Experiment No. 1.
[0156] The thus obtained glass-like carbon tube was found to have a
difference of 1.2 mm between the maximum outside diameter and the
minimum outside diameter. It is inferior in roundness to the sample
in Experiment No. 1 and hence it needs to be machined before it is
used as an inner tube.
Effect of the Invention
[0157] As mentioned above, the present invention provides an inner
tube of glass-like carbon for CVD apparatus and a process for
production thereof. The inner tube has its surface roughened
without increase in metal impurities which cause particles. The
roughened surface improves adhesion to the CVD deposit film. The
surface roughening does not give off dust. Because of these
features, the inner tube obviates the necessity of frequent
cleaning to remove CVD deposits therefrom. In addition, the
glass-like carbon tube has a high degree of roundness, so that it
is suitable for CVD apparatus.
* * * * *